A wide variety of IMDs have been developed over the years or are proposed that provide cardiac rhythm management of disease states manifested by cardiac rhythm disorders and heart failure. Implantable pacemakers have been developed that monitor and restore heart rate and rhythm of hearts that suffer bradycardia (too-slow or irregular heart rate), tachycardia (regular but excessive heart rate) and heart failure (the inability of the heart to maintain its workload of pumping blood to the body). Implantable cardioverter-defibrillators (ICDs) have been developed that deliver programmed cardioversion/defibrillation energy level shocks to the atria in response to detection of atrial fibrillation (rapid, uncontrolled heartbeats in the atria) or to the ventricles in response to life-threatening, ventricular tachyarrhythmias. Typically, single and dual chamber bradycardia pacing systems are also incorporated into ICDs. Implantable diagnostic and monitoring systems are targeted, for an emerging field that may change the way medicine is practiced. These systems typically monitor patients in their home environments, providing treating physicians with more complete information about their patients changing cardiac conditions. Proposals have been made to incorporate capabilities of pervasive computing into such therapy delivery and monitoring IMDs and the external medical devices employed to communicate with the IMDs and remote locations via the worldwide web.
These cardiac IMDs have traditionally employed capabilities of sensing the electrogram of the heart manifested by the cyclic PQRST waveform at one or more location principally to detect the contraction of the atria as evidenced by a P-wave meeting P-wave detection criteria of an atrial sense amplifier and/or the contraction of the ventricles as evidenced by an R-wave meeting R-wave detection criteria of a ventricular sense amplifier. The timing of detected atrial and ventricular sense events is used to ascertain normal sinus rhythm or the presence of bradycardia, tachycardia or tachyarrhythmia in the monitoring and therapy delivery contexts.
Among the earliest developed cardiac rhythm management IMDs were simple, single chamber, fixed rate pacing systems comprising an implantable pulse generator (IPG) and a lead bearing one or more pace/sense electrode adapted to be placed in contact with the heart chamber to be paced (commonly referred to as pacemakers) that provided fixed rate pacing to a single heart chamber when the heart rate fell below a lower rate limit. The earliest ICDs delivered a defibrillation shock to the ventricles when heart rate and regularity or morphology criteria were met. It was proposed that blood pressure sensors or accelerometers be incorporated so that the absence of mechanical heart function during fibrillation could also be detected to confirm the tentative determination of fibrillation before a shock therapy was delivered, but suitable sensors were not available.
Over the years, such pacemakers and ICDs evolved in complexity and capabilities as described in greater detail herein. Increasingly complex signal processing algorithms were developed and implemented in the effort to glean as much information as possible about the instantaneous state of the heart in order to provide the appropriate therapy to restore heart rhythm and to avoid mis-delivery of a therapy that would potentially harm the patient.
The accuracy of detection of atrial and ventricular sense events by sense amplifiers can deteriorate due to a wide variety of effects that distort the signal applied to the sense amplifiers such that the P-wave or R-wave is either not detected (an “undersensing” condition) or an atrial or ventricular sense event is mistakenly declared (an “oversensing” condition). The ability to distinguish a true sense event in a distorted signal is also complicated by the necessity of protecting sense amplifier circuitry following delivery of a pacing pulse or cardioversion/defibrillation shock. Conventionally, the sense amplifier circuits are disconnected from the sense electrodes during a blanking time period timed out following delivery of a pacing pulse or cardioversion/defibrillation shock to protect the circuitry. Longer refractory time periods are also timed out following delivery of a pacing pulse or cardioversion/defibrillation shock during which any sense event is declared to be refractory to avoid inappropriately restarting timing periods.
It has been recognized that other indicators of heart function, particularly indicators related to mechanical heart function, would be of great value in augmenting the algorithms that process atrial and ventricular sense events in order to resolve ambiguities that can inherently arise. It is also desirable to be able to ascertain whether a delivered pacing pulse has “captured” the heart, i.e., caused the heart chamber to contract. Further more, it is desirable to be able to rapidly determine that a delivered cardioversion/defibrillation shock has effectively terminated a tachyarrhythmia and that the heart has returned to normal sinus rhythm.
There are other situations where indicators related to mechanical heart function incorporated into pacing systems would be useful. Patients suffering from chronic heart failure or congestive heart failure (CHF) manifest an elevation of left ventricular end-diastolic pressure, according to the well-known heterometric autoregulation principles espoused by Frank and Starling. This may occur while left ventricular end-diastolic volume remains normal due to a decrease in left ventricular compliance concomitant with increased ventricular wall stiffness. CHF due to chronic hypertension, ischemia, infarct or idiopathic cardiomyopathy is associated with compromised systolic and diastolic function involving decreased atrial and ventricular muscle compliance. These may be conditions associated with chronic disease processes or complications from cardiac surgery with or without specific disease processes. Most heart failure patients suffer from symptoms which may include a general weakening of the contractile function of the cardiac muscle, attendant enlargement thereof, impaired myocardial relaxation and depressed ventricular filling characteristics in the diastolic phase following contraction. Pulmonary edema, shortness of breath, and disruption in systemic blood pressure are associated with acute exacerbations of heart failure.
These disease processes lead to insufficient cardiac output to sustain mild or moderate levels of exercise and proper function of other body organs, and progressive worsening eventually results in cardiogenic shock, arrhythmias, electromechanical dissociation, and death. In order to monitor the progression of the disease and to assess efficacy of prescribed treatment, it is necessary to obtain accurate measures of the heart geometry, the degree of heart enlargement, and the mechanical pumping capability of the heart, e.g., ejection fraction, under a variety of metabolic conditions the patient is likely to encounter on a daily basis. These parameters are typically measured through the use of external echocardiogram equipment in the clinical setting. However, the measurement procedure is time consuming to perform for even a resting patient and cannot be practically performed replicating a range of metabolic conditions. Typically, the echocardiography procedure is performed infrequently and months or years may lapse between successive tests, resulting in a poor understanding of the progress of the disease or whether or not intervening drug therapies have been efficacious. Quite often, only anecdotal evidence from the patient is available to gauge the efficacy of the prescribed treatment.
For these and other reasons, it has been proposed to employ sensors typically located in a blood vessel or heart chamber that respond to mechanical heart function to derive a metric that changes in value over the heart cycle in proportion to the strength, velocity or range of motion of one or more of the heart chambers or valves. It is desirable that such a metric would complement the measurement of the EGM to more confidently detect an arrhythmia or trigger delivery of a delivered therapy and to derive a number of indicators of intrinsic cardiac performance and response to a delivered therapy that can be employed to confirm or alter delivery of the therapy or indicate the state and progress of the underlying cardiac disease. Thus, permanently implantable sensors have been proposed and used to some extent to measure blood pressure, temperature, concentrations of various blood gases, and/or blood flow as blood fills and is ejected from a heart chamber of interest. A lead of the type described in commonly assigned U.S. Pat. No. 5,564,434 possesses capacitive blood pressure and temperature sensors as well as EGM sense electrodes that can be employed in this manner. Doppler flow sensors have also been proposed.
It has been proposed to employ permanently implantable sensors that provide a more direct measure of mechanical motion of muscle mass or particular structures of the heart, including one or more of valve opening and closing and the motion or expansion and contraction of the septal wall and the ventricular and atrial wall. Such sensors include intracardiac pressure sensors, accelerometers, impedance measurement electrode systems and Doppler motion sensors. In an approach related to monitoring rejection of heart transplants, a magnetic field responsive Hall effect device and a permanent magnet are implanted directly across the septum or a heart wall as taught in U.S. Pat. No. 5,161,540, and the Hall effect device is powered by an implantable generator and telemetry transceiver. The compliance of the heart wall is monitored to detect any loss of compliance characteristic of rejection of the heart transplant.
As noted in U.S. Pat. No. 5,544,656, measurement of myocardial wall thickness, as well as end-systolic and end-diastolic dimensions, are important in evaluating the effects of changes in regional myocardial function and contractility, including evaluating myocardial oxygen supply and demand, in acute and chronic animal studies. A transit-time sonomicrometry system is disclosed in the background of '656 patent that uses two piezoelectric crystals, one as a transmitter and the other as a receiver, and operates by measuring the time required for ultrasound to travel between the transmitting and receiving transducers. An advantage of this system is its ability to provide an absolute dimension signal output calibrated in units of distance. However, it is asserted that the two-crystal system has several disadvantages such as (1) it is necessary to insert a piezoelectric crystal through the myocardium, which can damage the myocardial nerve and blood vessel supply of the myocardial wall, (2) it is difficult to position precisely the endocardial crystal at the tissue/blood sub-endocardial interface, and (3) it can be difficult to maintain good alignment at all times throughout the cardiac cycle for short term and particularly during longer duration studies (>12 weeks). Tracking and non-tracking Doppler echo displacement systems and their purported deficiencies are also disclosed in the '656 patent.
The '656 patent then discloses a closed-loop, single-crystal, ultrasonic sonomicrometer capable of identifying the myocardial muscle/blood interface and continuously tracking this interface throughout the cardiac cycle using a unique piezoelectric transducer that operates in the manner of a Doppler echo sensor implanted at least partly in the myocardium and partly in the blood within a heart chamber. The echo sensing transducer disclosed in the '656 patent is characterized as overcoming these deficiencies in measuring myocardial wall thickness noted with transit-time sonomicrometry system and the tracking and non-tracking Doppler echo displacement systems. PCT publication WO 99/07285 also discloses an ultrasound echocardiography system particularly to measure the left ventricular performance using an array of ultrasonic crystals on a lead implanted in the right ventricle and aimed toward the ventricular septum and the left ventricular wall. T
Sonomicrometer systems that are installed epicardially about the heart to measure heart movement across a number of vectors are also disclosed in the article “Miniature Implantable Sonomicrometer System”, by Robert D. Lee et al., (Journal of Applied Physiology, Vol. 28, No. 1, January 1970, pp. 110–112), in EPO 467 695 A2, and in PCT publication WO 00/69490. The Lee article describes an implantable monitoring system attached to the epicardial electrodes. But, it is necessary to perform invasive surgery to expose the locations where sonomicrometer crystals are surgically attached to the epicardium.
Some of the various chronically implanted sensors described above are intended to be incorporated into lead bodies that are typically introduced transvenously into the relatively low pressure right heart chamber or blood vessels accessible from the right atrium through the patient's venous system. The introduction of such sensors into left heart chambers through the arterial system introduces complications that may be difficult to manage both acutely and chronically. The surgical approach to the exterior of the heart is also not favored as it complicates the surgery and recovery of the patient. However, measurement of left heart function is desirable in a number of clinical cases including chronic heart failure.
Moreover, despite many years of development and numerous proposals of various types of mechanical heart function or performance measuring sensors, there remains a need for suitable sensors located in a blood vessel or right heart chamber that responds to mechanical heart function to derive a metric that changes in value over the heart cycle in proportion to the strength, velocity or range of motion of one or more of the heart chambers or valves. The prior mechanical heart function sensors devices have been generally unsuccessful due to a.) the added complexity and reduced reliability of associated leads, and b.) the challenge of obtaining useful hemodynamic information from such surrogate measurements, which usually address only the right heart chambers. There remains a need for derivation of such mechanical heart metrics would complement the measurement of the EGM to more confidently monitor heart failure or myocardial infarction, detect an arrhythmia or trigger delivery of a delivered therapy and to derive a number of indicators of intrinsic cardiac performance and response to a delivered therapy that can be employed to confirm or alter delivery of the therapy or indicate the state and progress of the underlying cardiac disease.